The present invention relates to an electron beam apparatus for testing (or inspecting), observing, and evaluating the surface of a sample such as a semiconductor wafer for structure, electric conduction and the like, and more particularly, to an electron beam apparatus which can accurately and reliably evaluate a pattern formed on a sample with a minimum line width of 0.15 μm or less at a high throughput.
Conventionally, electron beam apparatuses have been known to irradiate an electron beam onto the surface of a semiconductor wafer, which is a sample under testing, for scanning to detect secondary electrons emitted from the wafer, generate image data of the surface of the wafer from captured signals indicative of the detected secondary electrons, and detect whether the image data matches or does not match each of dies on the wafer to detect defects on the wafer.
Such an electron beam apparatus also includes a projection imaging electron beam apparatus. The projection imaging electron beam apparatus forms an enlarged image from secondary electrons emitted from the surface of a wafer irradiated with a primary electron beam or electrons reflected from the same using a multi-stage lens system such as an objective lens. Since the projection imaging electron beam apparatus is capable of evenly irradiating an electron beam to a relatively large area on the surface of a sample, tests can be conducted at a higher throughput than that obtained using a SEM-based electron beam apparatus.
Japanese Patent Public Disclosure (Kokai) No. 11-283548 describes a projection imaging electron beam apparatus which directs a plurality of primary electron beams onto a sample from an inclined direction.
While the projection imaging electron beam apparatuses are typically capable of conducting tests at a relatively high throughput as described above, the following problems are implied in the projection imaging electron beam apparatuses.
When reflected electrons, rather than secondary electrons, are detected by a projection imaging electron beam apparatus, a test cannot be conducted at a desired throughput in some cases because a number of reflected electrons emitted from the surface of a sample is approximately 1/100 that of secondary electrons.
In some prior art examples, an ExB deflector is used to separate a primary electron beam from emitted electrons (secondary electrons or reflected electrons), but the use of the ExB deflector disadvantageously results in increased color aberration caused by energy diffusion of the secondary electrons or reflected electrons within an electric field and a magnetic field. Further, when a primary electron beam is directed using an ExB deflector, a resulting image is more susceptible to blur due to a spatial charge effect if primary electrons travel along an optical path close to that of secondary electrons.
Further, the apparatus described in Laid-open Japanese Patent Application No. 11-283548 directs a plurality of primary electron beams onto a sample in an inclined direction between an objective lens for enlarging or focusing emitted electrons and the surface of a sample, so that the emitted electrons are deflected by a draw-out electric field (an electric field for guiding emitted electrons toward a secondary optical system) to cause an exit angle to largely incline from the optical axis, resulting in a reduction in the number of emitted electrons per incident electron. Thus, this apparatus may fail to conduct a test at a desired throughput as well. Also, when a sample has ruggedness on the surface, the resulting image tends to include shading.